A321 Captain reported a pressurization ECAM and door open annunciation during climb. Crew elected to return to departure airport where maintenance personnel discovered a jammed pin affecting the door.

Narrative

After Departing ZZZ and climbing through 11500 the ECAM alerted us to an open door indication and at the same time we felt a pressurization change in our ears. I transferred controls to the first officer and proceeded to assess and manage the problem. The Ecam indicated to descend back down to 10000ft. We notified ZZZ ATC that we need to descend back to 10000ft and explained the situation. ZZZ center descended us back down and began vectoring us around their airspace. Since the airspace was so busy and we were definitely task saturated dealing with a non normal situation; I decided to use the SAT Call and talked to dispatch and Maintenance (MX) control to get them in the loop as quick as possible. I told Mx control about the problem. With the change in pressure we felt;the door indication and the fact that the Cabin ALT. Was pegged at -400 and wasn't changing; we agreed that we should return to ZZZ. We then needed to notify flight attendants and the passengers. At that point I called the flight attendants and told them of our situation and that it wasn't an emergency and to prepare the cabin for landing and that we were about 15 minutes out. I then made a PA to the passengers; delicately explained the situation and that we would be returning to ZZZ. We notified ATC of our intention and that we would need XXL and that we would be performing an overweight landing. I then proceeded to review the overweight landing checklist in the QRH. We chose XXL due to it being XXXXX ft long which would provide us with a longer roll out.. We told ATC of our intentions and they proceeded to vector us to XXL. After completing the checklists and set up for an overweight landing and ILS to XXL and briefing for both; We transferred controls back to me and I then proceeded to fly the Aircraft and performed the ILS to XXL. We safely landed on XXL with the vs no more than 50-100 ft per minute on touchdown and had med braking selected with max reverse. I taxied off the runway with no further incident. Upon Reaching the gate MX met the aircraft and while entering the write ups in the logbook I explained to them what happened. They then went and did an assessment of the door. A few minutes later MX came back and told us the one of the lower pins was indeed jammed in the down position. He showed us a picture of the pin and you could see that the pin was jammed in the down position. He explained that that would have definitely caused the scenario we experienced. I debriefed the #1 cabin crew member and he said that they were calling at the same time we called them to let us know that the door open" light was illuminated and that he was hearing a loud continuous "clicking noise" Coming from the Door."